Ultra miniature resonators for electron spin resonance: Sensitivity analysis, design and construction methods, and potential applications

This paper describes a recently developed new family of miniature surface resonators, used for electron spin resonance spectroscopy and imaging. The first part of the paper provides a detailed description of the operational principles of the surface resonators. It also includes sensitivity analysis for a variety of configurations with inner dimensions ranging from 150 m down to 2 m, operating at the Ku, Q, and W frequency bands. Most of the data presented here is based on theoretical predictions; however, some of it is accompanied by experiential results for verification. The second part of the paper describes a new type of double-surface microresonator and its production method. This new configuration enables an efficient coupling of the microwave energy from millimetre-sized microstrip lines to micron structures even at relatively low frequencies. The resonator is analysed both theoretically and experimentally-exhibiting ultra-high spin sensitivity. The conclusion of the two parts of the paper is that micron-scale surface microresonators may achieve spin sensitivity of a few thousands of spins in one second of acquisition time for special samples, such as phosphorous-doped ²⁸Si, at cryogenic temperatures. However, further miniaturization below 1-2 microns does not seem to be beneficial, sensitivity-wise. In addition to their high spin sensitivity, these resonators have a huge conversion factor, reaching in some cases to more than 500-1000 G of microwave magnetic field with input power of 1 W. Some possible applications of these unique capabilities are also described herein.